OP.GG retrieves data from the official League of Legends API server, and does not officially support data for custom games.
Therefore, the stats for custom games is not provided.
Currently, the queue types (modes) supported by OP.GG are as follows:
Normal (Quickplay) / Ranked Solo/Duo / Ranked Flex / ARAM / Co-op vs. AI / Clash / Arena / U.R.F / Nexus Blitz / Special modes